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  • IAN: All right, folks.

  • Welcome back to week six, where we're going

  • to talk a little bit about exposure.

  • So to this date we've talked a lot about composition, lensing, storytelling,

  • without focusing too much on the technical details of how

  • to actually make an image, and match it to our intentions.

  • So we'll dive a little bit more in that today.

  • And I think it's important to define exposure.

  • So the idea of exposure is that we want to render

  • a scene in a specific way using our camera controls to interpret

  • the amount of light in a given scene.

  • So what does that really mean?

  • Well we're going to dive deep into this, and we're

  • going to look at each of the different camera controls.

  • How they affect the image, and how we can utilize

  • them to make different exposures.

  • So what's our goal with exposing an image?

  • It's really to capture that intentional image.

  • It's to make a decision about how we want an image to look

  • before we press the shutter button.

  • When you have a camera that's on auto mode and you press the shutter button

  • you're basically giving up all the decision making to a small machine.

  • And they're sophisticated, sure, but they're not intelligent.

  • And our goal is for us as intelligent operators

  • to make the decisions for the camera, or at least

  • override decisions that may be poor.

  • So I think what we really need to understand

  • is what exactly is it that we're trying to do?

  • And so in any given scene there's some amount of light.

  • And we can use absolute measurements to tell how much that is.

  • It could be 150 foot candles of light, but I don't know what that means.

  • I sort of conceptually understand that, but it doesn't help me take a camera

  • and make an image.

  • I have to do a bunch of math.

  • It gets very confusing.

  • There's easier ways.

  • So what we really need is a relative amount of light,

  • or at least the scale--

  • a relative scale that we can use to adjust our camera settings.

  • So for us in this class when we're talking about photography,

  • and then later cinematography, we're going to use the concept of f-stops.

  • And all an f-stop is is a doubling or halving unit.

  • So if you have a camera that has some sensitivity,

  • and you double the sensitivity of it, that's one stop.

  • If you have a camera that has some light coming into it,

  • and you cut the amount of light coming into it by one half, that's one stop.

  • They're going in opposite directions, but it's

  • an equivalent unit of one stop.

  • So just briefly to get a sense of how this

  • might look when we look at an image we have here an image from Mount Auburn

  • Cemetery that's at exposure, roughly.

  • And I made this exposure using a DSLR, and the light meter

  • that was in the camera told me that if I set the camera settings to this

  • it'll be at exposure.

  • And I think it did a pretty good job.

  • There's no real complaints there.

  • So in an effort to sort of investigate how much difference a stop might be

  • I opened up one stop.

  • So I allowed double the amount of light to strike the sensor.

  • And you can see that it gets much brighter.

  • If we go backwards we have some dark shadow detail in here, neutral gray,

  • some bright white, and everything feels naturalistic.

  • We start to brighten up that neutral gray begins

  • to push towards the lighter gray, this definitely begins to clip,

  • these trees feel a little bit brighter.

  • Not completely unnatural, but getting there.

  • So now if we allow four times as much light in, or two stops--

  • remember that you're doubling each time you open up a stop.

  • So you double, and then you double again.

  • Now this image really starts to fall apart.

  • And this is the classic overexposure that maybe you've accidentally done,

  • or you've been struggling with your camera controls.

  • And you end up here where you have clipping elements, there's no detail,

  • there's no actual shadow detail.

  • No dark tones in this at all.

  • It's all light grays.

  • And if we go even further it completely falls apart.

  • And we could keep opening this--

  • allowing more light to hit the camera and eventually we'd

  • end up with a solid white image.

  • So, OK, that's overexposure.

  • But when we look at this image again--

  • add exposure.

  • This is the same image from the beginning.

  • And we go the other way, I've now reduced

  • the amount of light entering into the camera by a half, or one stop.

  • And you can see that all the tonalities get depressed.

  • They're getting pushed down into the shadow areas.

  • There's more detail in that white snow, which was much brighter before,

  • but now it's sort of a shade of gray.

  • And if we keep going two stops so four times less light

  • is reaching in the sensor you now see like beautiful detail in here,

  • but everything else is turning into a muddy, crushed, shadowy darkness.

  • And if we keep going it becomes almost unreadable.

  • So all that is is to say that we have these tools that

  • allow us to increase and decrease the amount of light,

  • and we do so by measuring it in doubling or having units.

  • So how do we measure light?

  • Well, we have a brief video online that we posted earlier this week, on Friday,

  • on light meters, which if you've all watched, great.

  • And if not we'll talk a little bit about as a refresher.

  • But there's a few different ways.

  • You can use a handheld light meter like this to measure light,

  • but I think more often than not what we'll use

  • is the internal light meter of the camera.

  • And so this will measure the amount of light that's striking the subject,

  • and reflecting back through the lens onto some sort

  • of light sensitive material.

  • And it will say if you set your camera to these values you'll

  • get a decent exposure.

  • So they can be hand-held or internal to a camera,

  • but all light meters are calibrated to expose

  • for an idea called 18% reflective gray.

  • And they do so in different ways.

  • So briefly, before we look at a couple different ways

  • they do that, 18% reflective gray is often sort

  • of colloquially termed middle gray.

  • When exposed properly it forms the middle value between absolute light

  • and absolute black in your image.

  • It's right in the middle.

  • So it's a very handy reference tone for us.

  • And it's usually found on a gray card--

  • something like this, that reflects back 18% of the light that strikes it.

  • Cool.

  • So before we get here let us quickly fire this guy up.

  • So this is the output of our camera right now.

  • If we take a middle gray card and we place it

  • in front of the camera well illuminated--

  • we'll fill the frame with it.

  • We can see that in the middle of this there's this histogram,

  • and sort of all of the image data is centered right in the middle.

  • If we set it for this white background you

  • can see that it's sort of drifted back to the middle as well.

  • And that's sort of interesting because we would

  • expect this tone to reflect more light.

  • So there is a bit of a trick going on with reflective light meters

  • that we need to pay attention to.

  • Reflected light meters always assume that every single element

  • that they're metering is middle gray.

  • So there's a problem when you meter something that is not

  • middle gray like this white background.

  • And so if we actually are to open up this camera

  • and allow a little bit more light in--

  • what am I doing here?

  • There we go.

  • I has auto ISO on.

  • So there's the shadow.

  • Let's get this shadow out of here so we can--

  • so now all of a sudden we brighten the image up by allowing more light in,

  • and this tone is rendering normally or as we would wish it to.

  • So we can also do the same thing if we have a black object.

  • You can see that the camera using the reflective meter

  • set this value to add exposure, which is lining up

  • that small cursor with the caret in that scale.

  • You can see that this is not black.

  • It's middle gray.

  • It's actually rendering this incorrectly because what it's expecting to see

  • is this middle gray card.

  • So what we need to do in order to offset that

  • is actually underexpose what the camera is reading.

  • So now when we check this the camera is like you're over three stops

  • underexposed, but this starts to look correct.

  • So we talked a lot about this-- thanks, Ben.

  • [INAUDIBLE]-- in the light meter video.

  • And we'll come back to this, but that is to say that in any given scene

  • the exposure suggested by a camera is made to be 90% accurate.

  • Most scenes have a mix of light values, dark values, and values in between.

  • So if you sort of assume that that all melds down to about middle gray

  • you can suggest an exposure.

  • And the meter will suggest an exposure, and then

  • when you expose for that you'll be pretty accurate.

  • But so this scene here is pretty much all highlights.

  • And my camera said if you set it to these settings

  • you'll get a properly exposed image, but it's not.

  • It's too dark.

  • So if we open up one stop.

  • Well, we're getting closer.

  • Snow starting to look like snow.

  • And if we open up one more stop maybe we're a little bit too far.

  • It's getting a little bit bright over in this area over here,

  • but it's rendering as a very bright white.

  • And sort of side by side you can see it here.

  • The metered exposure versus compensating by increasing our exposure one stop.

  • So this is what I mean to say when I say that we need to be a little bit more

  • intelligent than the machines.

  • They're very sophisticated.

  • They can measure all kinds of things, but they're still sort of locked in

  • and reference at specific values.

  • So we need to understand what those are so

  • that we can compensate, and sort of use our intention

  • to override the camera when it's making, essentially, a dumb decision.

  • Yeah?

  • AUDIENCE: If middle gray is meant to be halfway

  • between pure black and pure white how come it's 18% and not 50?

  • IAN: So it has to do with the sensitivity of--

  • or the way that human eyes render light, and because it's logarithmic.

  • So it's a power function.

  • So 50% is not quite halfway on a linear scale

  • when it gets transformed, I think, but essentially it

  • is because if you have say--

  • this is a good example.

  • If you have one light bulb, and you turn it on,

  • and you add another light bulb it's almost

  • like you've doubled the amount of brightness,

  • and it's a very obvious difference, but if you have 100 light bulbs

  • and you turn one more light bulb on it's such a tiny incremental difference.

  • So we're dealing with light on this power function scale

  • where we're doubling and we're halving, and so the value that gets us

  • to that middle gray is actually 18% and not 50%

  • because it's not a linear scale.

  • Which there's plenty of math, and I certainly won't do it justice,

  • but we can dive down that rabbit hole another time.

  • So we looked a little bit about what exposure is doing

  • and how we might fool ourselves with the light meter,

  • but what are the actual settings that I was just changing on the camera?

  • I was pushing and pulling at some buttons and dials,

  • and I was clicking some things, and the image was changing, but what was it?

  • So the three main camera controls.

  • The first one is ISO, which is the sensitivity of a sensor to light,

  • or a film stock, or any kind of medium that we're using to capture light.

  • The second is shutter speed.

  • How long do we let light strike that sensitive medium for?

  • In the case of DSLRs it's how long the shutter is open,

  • but maybe there are other mechanisms in play on other cameras.

  • And the final one is aperture.

  • So in every lens there's a diaphragm that opens and closes.

  • And depending on how open that is or how small that is it

  • lets in more or less light.

  • That's a control that we have at our disposal.

  • And so by using all three of these elements

  • we can control how much light reaches the photographic sensor,

  • and how sensitive that sensor is to light more generally.

  • So let's dive in a little to each of these categories because there

  • are different artifacts that happen when we change and control each one of them.

  • So first one, ISO.

  • So it, again, measures the sensitivity of a medium to light.

  • In digital cameras it's the sensor.

  • In old film cameras it's actually a type of stock, like film stock,

  • and they each have different ISO values.

  • And the sensitivity doubles and halves, which

  • is that unit of a stop, when the ISO value is doubled or halved.

  • And in this little scale down below there's some common ISO values.

  • A few of them you'll notice are bigger than the other ones.

  • Those are-- maybe you can call them major stops of ISO.